Photostability of a highly luminescent europium β-diketonate complex in imidazolium ionic liquids

Article abstract of DOI:10.1039/b506915g

A high quantum yield and an enhanced photostability was found for a europium(III) tetrakis(2-thenoyltrifluoroacetonate) complex after dissolving the complex in a weakly-coordinating imidazolium ionic liquid. The Royal Society of Chemistry 2005.

Full text of DOI:10.1039/b506915g

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Photostability of a highly luminescent europium b-diketonate complex
in imidazolium ionic liquids{
Peter Nockemann, Eva Beurer, Kris Driesen, Rik Van Deun, Kristof Van Hecke, Luc Van Meervelt and
Koen Binnemans*
Received (in Cambridge, UK) 17th May 2005, Accepted 7th July 2005
First published as an Advance Article on the web 3rd August 2005
DOI: 10.1039/b506915g
A high quantum yield and an enhanced photostability was
found for a europium(III) tetrakis(2-thenoyltrifluoroacetonate)
complex after dissolving the complex in a weakly-coordinating
imidazolium ionic liquid.
2
(Tf N 5 bis(trifluoromethanesulfonyl)imide) was enhanced in
comparison with alkali counter ions. Moreover, the potential
influence of different cations on the spectroscopic properties was
avoided.
Compound 1 has been prepared by deprotonating four
equivalents of the ligand 2-thenoyltrifluoroacetone in ethanol with
an aqueous solution of NaOH, followed by addition of one
equivalent of [HMIM][Br] in ethanol and the dropwise addition of
Lanthanide(III) b-diketonate complexes are widely exploited for
applications such as materials for flat-panel displays, luminescent
1
probes in bioassays, UV sensors and as laser materials. This type
of complex exhibits a high quantum efficiency in combination with
2
narrow band emission and a high color purity. Contrary to solid
3 2
one equivalent of EuCl ?6H O in ethanol. The solution was left to
stir overnight, after which a yellow precipitation of 1 had been
formed. Crystals suitable for X-ray diffraction were obtained after
slow evaporation from an ethanolic solution of the complex at
room temperature.{ The ionic liquid was prepared following
modified standard literature procedures for the synthesis of
inorganic lanthanide compounds, coordination compounds with
organic ligands are well processable and compatible with
polymeric host matrices, so that they are of interest as active
3
materials in new types of OLEDs.
8
A well known example of a highly efficient red-emitting
spectroscopic grade ionic liquids.
europium(III) complex is [Eu(tta) (phen)] (tta 5 2-thenoyltrifluoro-
3
The crystal structure of 1 (see Fig. 2) shows that the
europium(III) ion is surrounded by four 2-thenoyltrifluoroaceto-
nate ligands. The coordination number of the europium(III) ion is
eight, and the coordination polyhedron can be described as a
distorted square antiprism. No solvent molecules are coordinated
to the europium(III) ion. The acidic H(C2)-hydrogen of the
4
acetonate) which has a quantum yield of 36.5%. The photo-
chemical stability is an important factor to consider when the
application of lanthanide(III) b-diketonates in luminescent devices
is assessed. It has been reported that lanthanide(III) b-diketonate
5
complexes can be unstable towards long-term UV irradiation.
imidazolium cation forms strong hydrogen bonds in a range from
2
Ionic liquids are a new class of solvents which have attracted the
attention of academic as well as industrial researchers during the
˚ ˚
4
.336(6) A to 2.689(7) A to the oxygens of the [Eu(tta) ] moieties;
2
6
last few years. Ionic liquids exhibit useful properties such as
…
that is in analogy with the C–H X hydrogen bonding in
9
imidazolium ionic liquids.
electric conductivity, a negligible vapour pressure and a high
7
thermal and electrochemical stability. Some recent studies
Dissolving [HMIM][Eu(tta) ] in the ionic liquid [HMIM][Tf N]
2
4
does not affect the spectroscopic fine structure of the emission
demonstrate the use of room-temperature ionic liquids (RTILs)
8
in photochemistry and spectroscopy. Lanthanide-doped ionic
spectrum of the compound compared to a solution of the same
2
compound in acetonitrile (Fig. 3). The Tf N anions are known to
liquids can be regarded as new luminescent ‘soft’ materials.
Our approach was to find a complex that combines a good
shielding of the lanthanide ion with photosensitization by an
efficient light-harvesting antenna like 2-thenoyltrifluoroacetonate,
and which is soluble in an ionic liquid. We have selected
the 1-hexyl-3-methylimidazolium tetrakis(2-thenoyltrifluoroaceto-
2
1
0
coordinate very weakly to lanthanide ions. Apart from the fine
structure, a slightly lower absolute quantum yield W was measured
for the complex in the ionic liquid (54%) compared to a fresh
solution in acetonitrile (61%).
Although the freshly prepared solution in acetonitrile exhibited
the highest quantum yield W, after 10 days of exposure to daylight
4
nato)europate(III) complex, [HMIM][Eu(tta) ] (1), wherein the
coordination sphere of the lanthanide ion is saturated and thus no
solvent coordination occurs (Fig. 1). By introducing the 1-hexyl-3-
+
methylimidazolium ([HMIM] ) cation in the ionic complex, the
2
solubility in the corresponding ionic liquid, [HMIM][Tf N]
Department of Chemistry, Katholieke Universiteit Leuven, Celestijnen-
laan 200 F, B-3001, Leuven, Belgium.
E-mail: koen.binnemans@chem.kuleuven.be; Fax: +32 16 32 79 92;
Tel: +32 16 32 74 46
{
Electronic supplementary information (ESI) available: Experimental
details, characterization details of the complexes, table of lifetimes and
absolute quantum yields, synthesis of spectrograde ionic liquids. See http://
dx.doi.org/10.1039/b506915g
Fig. 1 1-Hexyl-3-methylimidazolium tetrakis(2-thenoyltrifluoro-aceto-
nato)europate(III) complex.
4
354 | Chem. Commun., 2005, 4354–4356
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Fig. 4 Comparison of the long-term photostability (exposure to day-
light) of europium(III) b-diketonate complexes in acetonitrile and in ionic
liquids.
Fig. 2 Part of the crystal structure of 1, showing the coordination sphere
around the europium(III) ion.
1
2
reported by Xu et al. The mechanism of the decay has been
proposed to be associated with a photodegradation of the
b-diketonate ligand, but no detailed mechanism has been reported
1
3
until now. Preliminary monitoring of the photochemical reaction
by NMR spectroscopy, that has been performed on the
corresponding lanthanum(III) complex in deuterated acetonitrile,
also indicates a decomposition of the b-diketonate ligands in
organic solvents. The long-term changes could also be followed by
UV/Vis absorption spectroscopy. The detailed mechanism of
degradation is not fully clarified yet and is still under investigation.
The luminescence lifetimes of the complexes are not significantly
affected by the degradation process.
Contrary to this photochemical instability, for a solution of 1 in
2
the ionic liquid [HMIM][Tf N], no significant decrease of the
quantum yield could be observed after 10 days of exposure to
daylight (see Fig. 5). The stability in the ionic liquid was also
monitored by exposing a fresh sample of 1 dissolved in
2
[HMIM][Tf N] to 8 periods of 1 hour (with periods of 5 min
Fig. 3 High resolution photoluminescence spectra of a solution of
[
HMIM][Eu(tta)
4 2
] in acetonitrile and in the ionic liquids [HMIM][Tf N]
and [HMIM]Br.
in a quartz cuvette, a decrease in the quantum yield to W 5 51%
was observed (see Fig. 4).§ A decrease in quantum yield was also
observed for some reference europium(III) 2-thenoyltrifluoroace-
tonate complexes [Eu(tta)
(CH NH ][Eu(tta) ] ([(CH
3
(H O)
2
2
], [Eu(tta) (phen)] and
3
+
[
2
)
5
2
4
2
)
5
NH ] 5 piperidinium cation) in
2
acetonitrile. All samples were measured intermittently during a
time period of 10 days and stored under the same conditions at
room temperature.
A much faster decrease of the quantum yield of 1 in acetonitrile
after UV irradiation points to photochemical degradation of the
b-diketonate complex. This instability of europium(III) b-diketo-
nate complexes towards light exposure has already been
5,11–13
reported.
For instance, for a similar europium(III) tetra-
kis(2-thenoyl-trifluoroacetonate) complex with a different cation a
half-life (decay to 50% of the initial value of luminescence intensity
under UV irradiation) of 36 hours for the pure complex has been
Fig. 5 Relative decrease of quantum yield under UV exposure in an
acetonitrile solution compared to a solution in a [HMIM][Tf N] ionic
2
liquid and to a sample not exposed to UV radiation.
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Chem. Commun., 2005, 4354–4356 | 4355

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˚
detector using Cu-Ka radiation (l 5 1.54178 A). The images were
interpreted and integrated with the program SAINT from Bruker,
between measurements) to a 12 mW UV-lamp (324 nm). This
photostability test was carried out in comparison with a solution in
acetonitrile and a sample of the latter without UV irradiation
15
C
42
H
35EuF12
N
2
O
8
S
4
, M 5 1203.92, monoclinic, P2 /c, a 5 12.4045(8),
1
3
˚
˚
21
b 5 18.6683(16), c 5 20.5603(14) A, b 5 97.167(5)u, V 5 4724.0(6) A ,
23
(except for excitation during measurements). The quantum yield of
c
T 5 100(2) K, Z 5 4, D 5 1.693 g cm , m(Cu-Ka) 5 12.060 mm ,
F(000) 5 2400, crystal size 0.5 6 0.2 6 0.1 mm, 8007 independent
reflections (Rint 5 0.1816). Final R 5 0.0843 for 4845 reflections with I .
the solution of 1 in the ionic liquid [HMIM][Tf N] stabilized at
2
about 98% of the initial value for the fresh sample—the slight
decay is probably due to traces of water in the ionic liquid, which
could be introduced by handling the sample and which quenches
2s(I) and wR2 5 0.2185 for all data. The structure was solved by direct
methods and refined by full-matrix least-squares on F using the
2
1
6
SHELXTL program package. Non-hydrogen atoms were anisotropically
refined and the hydrogen atoms in the riding mode with isotropic
temperature factors fixed at 1.2 times U(eq) of the parent atoms (1.5 times
for methyl groups). CCDC 271619. See http://dx.doi.org/10.1039/b506915g
for crystallographic data in CIF or other electronic format.
luminescence. The dry [HMIM][Tf N] ionic liquid is hygroscopic,
2
but due to the absence of vapour pressure water can efficiently be
removed.
In the ionic liquid [HMIM][Br], significant changes in the fine
§ The quantum yield measurements were performed by means of an
integrating sphere. The absolute error on the quantum yield values is about
¡0.5%. The tetrakis(2-thenoyltrifluoro-acetonato)europate(III) complex
was excited at a wavelength of 340 nm.
structure of the emission of [HMIM][Eu(tta)
7
4
] can be observed.
2
emission at 615 nm is a hypersensitive transition
5
The D
0
A F
that reveals changes of the coordination sphere of the
europium(III) ion. This can be seen in Fig. 3. Changes are also
indicated by the dramatic decrease of the quantum yield down to
1
M. H. V. Werts, R. H. Woudenberg, P. G. Emmerink, R. van Gassel,
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14% for the fresh solution in the [HMIM][Br] ionic liquid,
compared to 61% in an acetonitrile solution. The most probable
explanation for this behaviour is a partial exchange of the
b-diketonate ligands by bromide anions, thus resulting in a less
efficient energy transfer.
2 J.-C. G. B u¨ nzli and G. R. Choppin, in Lanthanide Probes in Life,
Chemical and Earth Sciences – Theory and Practice, Elsevier,
Amsterdam, 1989; C. G o¨ rller-Walrand and K. Binnemans, in
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Gschneidner, Jr. and L. Eyring, North-Holland, Amsterdam, 1998,
vol. 25, ch. 167, p. 101.
The reason for the improved photochemical stabilization of
[
HMIM][Eu(tta) ] in the ionic liquid [HMIM][Tf
4
2
N] is still not
] leads to
understood, but the crystal structure of [HMIM][Eu(tta)
4
the assumption that hydrogen bonding between cation and ligands
could play a significant role, since it encapsulates the complex. A
strong interaction between host medium and guest is known to
remarkably influence the photophysical and photochemical
3 R. Reyes, M. Cremona, E. E. S. Teotonio, H. F. Brito and O. L. Malta,
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12
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5
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14
performed (similar to the studies by Chaumont et al.).
6
Nevertheless, along with the versatile properties of ionic liquids,
this system appears to have the potential to be developed to a new
class of luminescent ‘soft’ materials.
7
8
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In summary, we found that the 1-hexyl-3-methylimidazolium
tetrakis(2-thenoyltrifluoroacetonato)europate(III)
complex
is
photochemically stabilized in a weakly-coordinating imidazolium-
based ionic liquid. The tunable properties of ionic liquids together
with the stabilization of b-diketonate complexes can open up new
potential applications for these materials, e.g. as laser dyes or as
materials for emissive displays. Currently we expand our research
on the stability of tetrakis(b-diketonate) complexes in ionic liquids
3
+
towards other lanthanides emitting in the visible region like Tb
3+
and Sm , as well as towards the lanthanides emitting in the near-
3+
2
004, 16, 4063.
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3
+
3+
infrared region (Er , Nd , Yb ). The preliminary results are very
promising.
1
1
KB, KD and RVD thank the F.W.O.-Flanders (Belgium) for a
Postdoctoral Fellowship. PN is a Postdoctoral Fellow funded by
the EU (GROWTH No: GRD2-2000-30346 OPAMD). Financial
support by the F.W.O.-Flanders (G.0117.03), by the K.U. Leuven
11 L. D. Carlos, C. De Mello Donega, R. Q. Albuquerque, S. Alves, Jr.,
J. F. S. Menezes and O. L. Malta, Mol. Phys., 2003, 101, 1037.
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2
000, 38, 351.
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(
GOA 03/03) and by the EU (GROWTH No: GRD2-2000-30346
1
OPAMD) is gratefully acknowledged.
1
1
5 SAINT, version 5/6.0, Bruker Analytical X-ray Systems Inc., Madison,
Notes and references
WI, 1997.
16 SHELXTL-PC, version 5.1, Bruker Analytical X-ray Systems Inc.,
Madison, WI, 1997.
{ Crystal data for compound 1: crystals were grown from ethanol, intensity
data were collected on a SMART 6000 diffractometer equipped with CCD
4
356 | Chem. Commun., 2005, 4354–4356
This journal is ß The Royal Society of Chemistry 2005